Allow users to replace ffmpeg command with a custom path

[ubershmekel]

I don't know why, but just setting process.env.path didn't work for me when building on Netlify.

You can see what troubles I got into here: https://github.com/ubershmekel/ld47/blob/master/build/build-sounds.js

I ended up copying the audiosprite.js file and modifying it to allow using a custom ffmpeg binary file path.

[endel]

Created a PR to address this #83

(In the meantime, you can use @gamestdio/audiosprite package with this feature)

[tdukart]

I didn't see #83, but I took a different approach with another PR, #85.